This invention relates to a method of generating a signal indicative of the position, within a sequence of data elements of first and second kinds, of that element or pair of elements which lies at the centre of the or each largest group of elements of the first kind. The invention also relates to apparatus for carrying out such a method.
It is sometimes necessary to determine, within a sequence of data elements of first and second kinds, the position of that element or pair of elements which lies at the centre of the or each largest group of elements of the first kind. For example, in passive direction-finding radio receivers which employ an antenna of a multi-beam type, for example a so-called "Luneberg lens antenna" as described in an article by K. E. Cassel entitled "A Broad Band ESM/ECM Antenna with 360.degree. Multibeam Coverage" in Military Microwaves Conference Proceedings, 1982, the antenna has multiple feeds which form a large number of overlapping reception beams which give multidirectional (possibly even omnidirectional) coverage. The sensitivity S of the antenna as a function of direction 0 may be of the form illustrated in FIG. 1 of the accompanying diagrammatic drawings, where the successive curves 1, 2, 3 etc relate to the respective successive feeds. If this is the case and, for example, a radio source is located in the direction 0.sub.1, outputs, albeit of mutually different amplitudes, will be obtained from feeds 5-12 (possibly even more iff appreciable side-lobes are present), with the highest outputs coming from feed 8 and 9. The position 0.sub.1 of the source could, of course, be ascertained by comparing the output amplitudes of the various feeds, thereby ascertaining that in fact the highest amplitudes are coming from feeds 8 and 9 (corresponding to directions 0.sub.2 and 0.sub.3 respectively). However, this becomes impracticable when the number of feeds is large, for example ninety-six, and is also liable to lead to inaccuracies with sources of differing frequency. A better way when the number of feeds is large is to ascertain whether the output from each given feed is above or below a suitable threshold level (which level is the same for each feed) giving in effect a sequence of binary digits corresponding to the respective feeds. Such a sequence may be, for example, as shown in FIG. 2 of the accompanying drawings, where it is assumed that the antenna is omnidirectional, that there are ninety-six feeds the directional pattern corresponding to each of which includes a pair of side lobes of appreciable amplitude, that the source is in the direction 0.sub.1 (FIG. 1), and that a binary "1" corresponds to a signal amplitude from the corresponding feed which is above the threshold. If now the feed(s) corresponding to the centre of the largest group of "1"s is/are ascertained, this/these will in turn be an indication of the direction 0.sub.1 of the source. In the absence of the side-lobes, and hence of the "1"s corresponding to feeds 22-24 and 89-91, this could be done by finding the average of the numbers in the sequence of all the feeds to which a binary "1" corresponds, i.e. (5+6+7+8+9+10+ 11+12)/8=8.5. However such an averaging process can take quite an appreciable amount of time, and moreover only gives an accurate result in the absence of side-lobes. It is an object of the invention to mitigate this disadvantage.